The Hedgehog (Hh) signal transduction pathway regulates cellular processes such as cell migration, proliferation and differentiation in embryo development. Unsurprisingly, dysregulation of Hh signaling is implicated in oncogenesis;mutations to regulatory pathway components are linked to medulloblastoma and basal cell carcinoma, the most common brain and skin cancers, respectively. Furthermore, upregulated Hh pathway activity has been observed in other cancers, including prostate, breast and lung. A recent hypothesis suggests that oncogenic events in other signaling pathways may result in improper activation of Hh signaling, and support cancer progression. If true, this hypothesis suggests that the Hh pathway may present a therapeutic opportunity in the treatment of many cancers. Recently, our lab identified three components of a signaling pathway whose loss resulted in increased Hh pathway activation. These three proteins: LKB1, SIK1 and SIK2, are members of the LKB1/SIK signaling cascade. LKB1 itself is a tumor suppressor gene, whose loss results in the genetic disease Peutz-Jegher's syndrome. This potentially represents a novel mechanism for regulation of Hh signaling, and may also suggest a model for how an oncogenic event in a distal pathway could result in dysregulation of Hh signaling. In other words, a model for the general upregulation of Hh signaling in cancer. There are two aims: first, to understand which members of the LKB1/SIK signaling pathway participate in Hh pathway regulation. Second, to characterize the mechanism by which LKB1/SIK signals cause Hh pathway activation. To accomplish these aims I will study the effects of loss, overexpression and mutation on LKB1/SIK pathway components in cell culture. I will also examine the physiological consequences of these perturbations in zebrafish, to understand the in vivo relevance of these observations. I will then epistatically map the effects of LKB1/SIK pathway perturbation relative to known Hh pathway components, by modeling LKB1/SIK loss in cell lines with deletions for individual Hh proteins. In this way I hope to systematically determine the identities of the LKB1/SIK pathway components involved, and the identities of their partners in the Hh pathway. Public Health Relevance: This proposal investigates a novel observation linking two cancer-relevant biological pathways. In understanding this connection, we hope to identify a general pattern in the progression of some cancers which may suggest novel strategies for the treatment of disease.